Temperature sensitive control for electric blankets



Allg. 8, 1950 R A 'YORK ETAL 2,518,108

TEMPERATURE SENSITIVE CONTROL FOR ELECTRIC BLANKETS Filed June 21, 1947 2 Sheets-Sheet 1 TEK Aug. 8, 1950l R. A. YORK ErAL TEMPERATURE SENSITIVE ONTROL FOR ELECTRIC BLANKETS Filed June 2l, 1947 2 Sheets-Sheet 2 Patented Aug. 8, 1950 TEMPERATURE SENSITIVE CONTROL FOR ELECTRIC BLANKETS RaymondA A. York, Pitman, and Harry J. Woll, Audubon, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application June 21, 1947, Serial No. 756,138

(Cl. Zhi- 46) 5 Claims. 1

This invention relates to improvements in temperature-sensitive control circuits and, more particularly, to improvements in fail-safe apparatus which may be used for switching an electric heating current on and oit in response to information supplied by a temperature-sensitive device which preferably has a high negative temperature coeiiicient of resistance.

Heating devices for many purposes and having various types of thermostatic control elements to prevent overheating are Well known in the art. Thermostats have been developed which will open and close an electric circuit when they are subjected to quite narrow variations in temperature. Coupled with these have been means for proven"- ing damage to equipment when various parts of the apparatus failed mechanically. One type of apparatus having more than usually strict requirements in this respect has been the electric heating pad or blanket since it is necessary that this article be as free as possible from defects which Would introduce a rire hazard or subject the user to too much heat.

Electrically heated pads and blankets in use at present may be classified according to their heating controls as falling under two general types. The first is that type which has a temperature-sensitive element which responds to changes in the temperature of the ambient atmosphere. and a small piece of resistance wire enclosed in a protective box having openings therein. This box is placed in the room in which the article is used. The thermostat is set to respond at some desired temperature and when the ambient temperature drops to this point the heating circuit is turned on. Currentis supplied simultaneously to both the resistance Wire in the box and the heating circuit in the blanket. When the resistance wire in the control box has raised the temf perature of the air around the thermostat a predetermined amount, a set of contacts is opened and the current shuts off automatically. When the temperature within the box drops down sufficientiy low, the contacts are closed and heating current is again supplied. While simple in operation, this type of control has several undesirable features. Sudden drafts may cause false respouses and discomfort to the user. Also, no provision is made for accommodating more than one person under the same blanket except numerous trials made by the user until he nds the proper setting required to remain comfortable. It is also apparent that some further provision must be made to prevent excessive heating should the This type generally has a thermostat .J

thermostat contacts stick in the closed position. In practice, it has .been found necessary to include several more thermostats of rather bulky construction placed at various points in the heating circuit of the blanket to out off the current when excessive temperatures are reached. When operation of the heat control unit is faulty these may chatter and upon them is thrown the entire responsibility for safeguarding against the effects of undue amounts of heat until the failure is noted and steps taken to correct it.

Another general type of heating blanket or pad is that having a temperature-sensitive device embedded in the web of the article. The most satisfactory of these have utilized wires having a positive temperature coeflicient of resistance as one arm of a bridge circuit and a high gain vacuum tube amplier for switching current on and oli in the heater circuit in response to resistance changes in the heat sensitive material. While these are, in general, more stable in their operation than the type previously described, their safety features have not been such as to reeeive unqualified approval. Moreover, they have sometimes used vacuum tube fuses to protect against the effects of sticking relays and these tubes are expensive to replace when blown out. In addition to the unsatisfactory safety features, another undesirable characteristic has been the fairly low degree of sensitivity of the temperature-sensitive medium which they have utilized.

One object of the present invention is to provide an improved fail-safe circuit for controlling either a mechanical or electrical operation in response to a varying resistance.

Another object of the present invention is to provide an improved fail-safe circuit for controllingv a switching means in response to changes in the resistance of an element having a high temperature coecient of resistance.

Another object of the present invention is to provide an improved fail-safe circuit for controlling a switching means in response to changes in the temperature of an element which has a high temperature coeiiicient of resistance.

Another object of the present invention is to provide an improved control circuit for a heating unit which is responsive to changes in the 'teniperature of the unit.

Another object of the invention is to provide an improved control circuit employing temperature-sensitive element for a heating unit which will fail safe as the result of occurrence of either short or open circuits in the temperature-sensitive element.

Another object of the invention is to provide an improved control circuit for a heating unit which utilizes a temperature-sensing device which has a high degree of sensitivity.

Another object of the invention is to provide an improved control circuit for a heating unit which may utilize an element having either a negative or a positive temperature coeicient of resistance as the temperature-sensitive element.

These and other objects will be more apparent and the invention will be better understood `by reference to the following specification and to lthe accompanying drawings wherein:

Fig. 1 is a circuit diagram of a preferred embodiment of the invention,

Fig. 2 is a detail view of a part of thecircuit of Fig. 1 illustrating one form of relationship between the heating element and the temperaturesensitive element,

Fig. 3 is a graph of thyratron grid voltage versus thermistor resistance as applied to the thyratron tube shown in the circuit of Fig. 1 with certain circuit elements omitted,

Fig. 4 is a graph showing the effect on the voltage curve of Fig. 3 produced by an added D.C. voltage,

Fig. 5 is an alternative circuit which may be substituted for the circuit shown in Fig. 1,

Fig. 6 is a detail view of part of the circuits of either Figs. 1 or 5 showing an alternative type of switching device, and

Fig. 7 is another detail view illustrating an alternative type of control operation which may be carried out with the aid of the circuits shown in either Figs. 1 or 5.

As shown in Fig. 1, a heating unit 2 is supplied with current from the A.C. power lines. .A relay 4 with a capacitor 9 in shunt therewith and having contacts inseries with the heating unit 2 and with the A.C. lines serves as a switching device to turn the current flow on and off through the resistance wire of the heating unit. Control of current through the coil winding of the relay 4 is accomplished by placing a gas-filled thyratron tube 5, which may be a type 2D21, in series .l

with the relay winding and the A.C. lines. When the thyratron tube 6 is conducting current, the relay contacts are closed and current iiows in the wires of the heating unit. During extended periods when the thyratron tube is not conducting,

the relay contacts are open and no current flows in the heating unit. This does not mean, however, that the relay contacts open during each half cycle when the filament of the thyratron is positive and its plate negative. The relays moving parts have considerable inertia and the capacitor 9, which is connected across the relay coil, holds a charge during the negative half cycles which also tends to keep the relay contacts closed.

The improvements constituting the present invention have been designed to automatically maintain the temperature of the heating unit at a predetermined value in accordance with the desires of the operator and to cut off all current ilow in the heating unit if the resistance of the sensing device should travel either way outside a certain specified range.

As further shown in Fig. 1, there are placed in series across the A.C. power lines a thermistor I0 having a high negative temperature coeicient of resistance, a variable resistor I2, and a ixed resistor I4 which, together, operate as a potential divider. The thermistor II is placed advantageously within the medium I6, the temperature ient to a heating device which must be safe of which it is desired to control by the heat of the heating unit 2. This is shown more particularly in Fig. 2.

Referring again to Fig. 1, a transformer I8 has its primary winding 20 connected across the A.C. supply lines and has two secondary windings 22 and 24 wound in opposed relation to each other. One side of the secondary winding 22 is connected to one of the cathodes 26o of a double diode vacuum tube 28 and the other side is connected between the Variable resistor I2 and the fixed resistor I4. The anode 26a of the same half of the double diode tube 28 is connected in series with a capacitor 8, the other secondary winding 24 of the transformer I8, and the end of the thermistor I D opposite the end which is connected to the variable resistor I2. There is thus formed a series circuit consisting of the windings 22 and 24 of the transformer, one-half of the double diode 28, the capacitor 8, the thermistor I0 and the variable resistor I2. The secondary winding 22 thus provides a voltage in opposition to that normally imposed across the thermistor I 0 and variable resistor I2 in consequence of their being -connected across the A.C. power lines. The fixed resistor I4 supplies a certain xed current to the thermistor and variable resistor.

The other half of the double diode tube has its cathode 30o connected to the rst anode 26a and its anode 30a connected to one side of the A. C. supply through a resistor 32.

The control grid of the thyratron tube 6 is connected through a circuit comprising a resistor 34 and capacitor 36 in parallel to a point between the capacitor 8 and the anode 26a of the double diode vacuum tube.

The general operation of the above described control circuit is such that the control unit may be set to cause current to flow through the switching device when the resistance across the thermistor reaches or exceeds some selected value within a predetermined range of resistance but will maintain the switching device in an oi condition when the resistance of the thermistor is outside the predetermined range either above or below. This is one of the novel characteristics of the present invention and the means for accomplishing it will be more specifically described later. Since the thermistor is supplied with a substantially constant current, the potential across it varies only with its temperature. Thus, the switching device may be turned on when the temperature of the thermistor reaches or exceeds a certain selected value within a predeterminedv range and will not turn on when the temperature is outside the limits of that range. If the switching device is being used to connect to a current source a heating element which is supplying heat to a body in which there is located the thermistor in heat transfer relation to the heating element, the temperature of the body may be controlled within the predetermined range by the setting of the control unit.

Another significant feature of the invention resides in the utilization in a thermostatic control of a substance having a resistance which has relatively high temperature sensitivity, and the fact that it has been adapted to the present purpose, even though it has a negative temperature coeiicient of resistance. Examples of substancess suitable for the purpose are silver sulphide and boron. The problem in adapting a material having a negative temperature coeffiagainst mechanical wfailures; not only forfshort `circuits butifor openA circuits, ilesfinitheiact that if :an-open circuit occurs in'the.xthermistor..zcir cuit the condition normallyacalls forimorel'cur- `rent to fbe suppliedzto'zthezheating .element instead of beingturn'ed Ioffas safetyrequiresf However, in the controli circuitioftzthe present `invention', a more than normal increase'.inxresistanca suchas anfopen` or break in; the thermistorr. circuitgire- Lsultsinlthe thyratron 11otfbeing--iableifto :'re andhence no closingbf theerelayl contacts 'and no :current flowing inxzthe heatingV element.

.f More speciiically,v ,thex'operationa ofthecontrol circuit isas follows:

rIhe variable resistor; I2 islrstselected. to prof. vide for operation within azcertain desired. range of temperatura/say 70?-,1-10? F. .fr Inxtheexample shown in'Fig'., 1; thefvalueiof thismesistor may be varied between 0. and 40Go-ohms. )Thisresistance mayxbe calibrated so. that '.the. scalepresented to y the :user reads'in degrees 'or' temperature. The y'user'.sets'the dial for thetemperature at which he .wishesto usetheheater: and vswitches on the A,C. vline current. The thermistor Hlwhich is embedded in thezrn'edium` which is to be heated,l as say a blanket; has a .resistance whichdepends on' itsxtemperature. tInythe practical example chosen and for thecircuitsasshown', this elementfhas a resistanceof about 7000 ohms at 70 F. and 3000 ohmsat 110'F.

Referring'now to'Fig. 4,: when the combined resistance of the .thermistor l and variable resistor fl2is sufficiently high, the current conducted by the lower orv control half of the-double diode will be `sma1l,.uthe consequent negative charge on the .capacitor 8 will be lowered and the potential of ythefcontrol grid of thyratron E will rise above its'fcritical Value, as .indicated by the reference line, which results inthe thyratron ring and current being supplied to the heating` unit because of. closure of thecontacts of relay 4. For example, circuit values-maywbe'chosen such that the thyratron lires when. the'cornbinecl resistance of the thermistonzand Variablet resistor .equalsf 7000. ohms.u Iffrthegvariable:resistor is set rvatitslowest value; i. .e.-, zero.ohms,nthe'resistance ofthe thermistor will havepto'be .at itsxmaximum (7000 ohms) :in-.order tocscauser the` thyratron to fire. This `condition;y occurs; when/:theftemperature of the :thermistor isflv .ture 'offzthewthermistor ishigherthan" its resistance willlbe lower*.thana'lOOalfohmsand the `total resistance 'offtherrnistorand Variablere sistor 'willfthereforerbe .too lowzto causethe thy- .ratron tondre...A Asthevariable .resistor isf-set for` :increasing -rvalues` .of;=;resistance'fthe .resistance needed to :be contributed by;the1stherrnistor. is l-ess,1or,.in .other words,litsctemperature maybe higher' with the" thyratron. stillniin :condition Hto re. l The curve ofyFigl. 4 shows"thelrelationship between Ithe combined vresistance of the'.l thermistor l and variable -re'sistor;and 'the ringaofthe thyratron. .1W-ith the variable resistor-1 set at a predetermined value.' the; combined'resistance` i finali:

creases l,only as the resistanceloi-the thermistor increases,v that is,k as its .temperature fdecreases. 4 .The thyratron Ycontrol. grid f voltage rises, mean- "while, v.until it.A exceeds the :critic alvalue at which the; tube hres. .s vAll :points on thetcurve' of .thyrai r.: tron controLgridvoltage above'thexeference line represent values sumciently .highztorcauselthe gas tubetoconduct. :The :operatingzpoint when using a. thermistoryhaving. a ynegative :temperature;coeiilcientfl oftresistance. is; as shown' in Fig-;

-,:ure 4,.:.at that point .where` their-rising .valuepf .thyratron control 'gridivoltagecrosses the reference linev Oreritical potential.r A change-@in `:temperature oflnoV morethan a degree either' fway will cause the resistanceaof lthesthermistorto change sufciently to either-ni'e'or de-re the thyratron as the case maybe.

All points oni'the. curveof thyratron gridfvolt- 1 '.age. from. the'4 low 'point' of Vther'rnistor resistance 10 i upto `:the peak of the curve also represent points for whichthesumlof thevoltages acrossthe thermistor.y and the variable` resistor l 2- isf less zthanthe resultant of the voltages-acrossrthe transformer secondaries. The .peak fof the grid l voltage Vcurve is a null point at which thesum of. the resistor and thermistor voltagesand-'the transformer voltage are equal. All pointsonv `the .f grdfvoltage'curve beyond the-peak vrepresent points atfwhich the transformerfvoltageis exceeded @by the `resistor 'and thermistor voltages.

When the'coinbined potentialvacross' thevarii.. able resistor i Zand thermistorl yjust equals Athe resultant voltage across the transformer-"secondaries, almostno current o'ws 1in the-lowerhalf flfof thediode. The upper'. half 'of the diode `fcharges vthe capacitor 8 with a sufficiently high positive potential to provide the thyratron grid with a maximum positive --potentialfand vthus keep it conductive. -On either side vofl the peak,

v thelower half of the diode is conductingA current andthe more current thishaliconducts the -less y positive is the charge on capacitor S and'on'the control grid of the' thyratron until Ifinally the potential drops' .belowfthe critical or'ring potential.

Due principally to the presence'of resistor-5514, the therm-istorfis-fed with a substantially-'constant current from the ALC. supply and the voltage-'across it is directly proportional to its rre- 40y sistance. In series 'with this 4first-voltageacross the thermistoris Vone of opposite polarityv derived from the secondary windings 22 and i214 of .transform/51H53. y These secondary Awindings Vare,

first oi-all, woundhso that'ftheirf-voltagesnppose each other and ltire-voltage which they superimposeon the tlierrnistor is actually -the diilerence between? thetwo. This.l resultant,voltage isgin turn3 opposed to the .voltageacross the thernrlstor derived'- directly froml the` .AJ-C. supply. "When this voltage is rectified by the lower lhalf ofthe 'double diode 28 thereY is obtained a Dea-voltage which varies with" the resistance'of the' .thermistor, when the variable resistor" is Yset at a constant value, as shown in Fig. 3.

The upper half ofthe double diode 28falso rectifles the alternating current'irom the line, and the resistor 32.in series with the anode-of the upper half of the-diode causes a positive D.C. voltage to be added to that obtainedlfrom lthe w lower half and this added voltage issuch .asfito lower the'reference pointfand allow.' the thyravtron toire whenever the resistance of-'the thermister exceeds the set'valuaprovided it does not lexceed it too much, as in'anopen circuit, -Fig. 4 v illustrates this.

When the `temperature "of the" .thermistor J exceeds the value' for which thevariable'f-resistor i2 has been set; itsresistance will have fallen .to a value such that-the potential across the lower 270 half of the diode rises andthe upper plate ofithe capacitor 8 is sufficiently fnegativethatv the thyratron is prevented from conducting bythe 'corresponding negative potentialon itscontrolgrid, -One the fsafety .features :of theA above. cir- AT fcuitis -that if th'e thermistor.: should'f-failwby open circuit, a high negative charge is placed on ca- A pacitor 8y during the rst subsequent half cycle when point A isv negative and this prevents conduction of the tube during the next half cycle when point A is positive.

The transformer secondary winding 24 also has a protective function in the circuit. It supplies a suicient A.C. bias to the grid of` the thyratron, the grid being negative when the plate yis positive, to prevent the thyratron from firing if the double diode tube 28 should fail. The presence of this bias also prevents the thyratron from drawing anode current until its cathode has had time to warm up.

A capacitor 36 may be placed in parallel with .l grid resistor 34 to prevent the grid from causing the thyratron to fire too soon. Due to the diode action of the thyratron grid, when the grid is positive with respect to the cathode, this capacitor 36 prevents the thyratron from firing during the part of the 60 cycle wave when the grid bias is approximately zero. This function is used only when the double diode is inoperative.

Grid resistor 34 is included so that the thyratron does not load the circuit too heavily and also to regulate the current drawn by the grid of the thyratron.

Although the circuit has been described in connection with use of a thermistor having a negative temperature coeihcient of resistance, it is obviously also operative when a material having a positive coefficient is used. The operating point of the thyratron will then be on the down slope of the curve at the critical potential.

temperature sensitive resistor is responsive. For

example, the resistor may be in a furnace wall l and the switching device may be placed at a distance opening a valve for admitting material to the furnace or operating a warning light or a recording device.

As an alternative to using a relay as a means for switching line current on and off in the heating unit 2, a saturable reactor d2 may be used as shown in Fig. 6. In this modication, a re- ;actor 'A2, having two windings wound in opposed relationship and having a capacitor d4 connected; across the primary windings, is substituted for the relay il. When the thyratron tube 6 is not conducting, the current through the circuit containing the heating element 2 and the transformer primaries is normally limited by the high inductance of the transformer. But when the tube is conducting, the direct current in the secondary lowers the inductance in the primary windings so that current in the heater circuit is no longer limited to the same extent.

Another form of control circuit which may be used in the present invention is illustrated in Fig. 5. This circuit is a form of parallel-T network and results in the same type of peaked thyratron grid voltage curve, with relation to the resistance of the thermistor, as shown in Fig. 4. In the modification, two branches of a parallel circuit are. connected across the A.C. mains,

one end being connected tothe line through az.

resistor 46. One branch of the circuit consists of a capacitor 48, the variable resistor |'2 and thermistor I0. The other branch, in parallel, consists of a resistor 50 and capacitor 52. Each of these branches is connected in parallel to the lower cathode 26o of double diode tube 28, the first branchbeing connected to the cathode between ,the capacitor 43 and variable resistor I2 through a capacitor 54 and the second branch being connected between resistor 50 and capacitor 52 through a resistor 56.

The line transformer 58 has buta single secondary winding'from which a 2 v. bias is tapped to apply a bias to the control grid of the thyratron 6. As in the case of the circuit shown in Fig. 1, this bias prevents the thyratron from ring when the double diode is not conducting.

The operationof this circuit is substantially the same as that shown in Fig. 1. The curve of thyratron control grid voltage as related to the combined resistancer of thermistor Il) and Variable'resistor I2 is exactly the same as that shown in Fig. 4. This circuit also has protective features for either shorts or opens in the thermistor circuit. When the former fault occurs, the high resistance path through the resistors 16, 50, and 56 results in the thyratron control grid voltage dropping below its critical value. Should an open circuit occur, the path through the capa-citors 48 and 54 is utilized to maintain a high negative potential on the thyratron control grid during that portion of each cycle when the cathode of the gas tube 6 is negative and the plate positive.

There has thus been described an improved circuit for controlling either a mechanical or an electrical operation in response to changes in the resistance of a control element. The resistance of the control element preferably is sensitive to l temperature changes and the operation which is controlled may be one which supplies heat to a medium with which the temperature sensitive element is associated. l

We claim as our invention:

1. An apparatus comprising a source of A.C. current, current switching means connected to said source through the anode-*cathode path of a grid controlled electron discharge device, means controlling the potential of said grid, said controlling means including a temperature responsive resistor in heat transfer relation to a medium adapted to be heated, means applying a -iirst A.C. potential acrosssaid resistor, said first potential being Variable in response tothe temperature of said resistor, means applying in series with said first potential a second A.C. potential of predetermined value and opposite in sign to said rst A.C.'potentia1, means rectifying the potential which is the resultant of said first and second A.C. potentials, means applying said resultant potential to said grid such that said grid is negative, means applying an additional positive D.C. potential to said grid in opposition to said rectified resultant potential, said additional potential being sufficiently high to maintain said discharge device conductive when said iirst A.C. potential is within a predetermined range of numerical values above and below that ofsaid second A.C. potential, and also being suiciently low to permit the lowering of the potential `on said grid below a value necessary to maintain said discharge device conductive when said rst A.C. potential is outside said-range.

2. Apparatus according to claim 1 and including means for applying an additional A.C. biasing potential to saidgrid of said dischargedevice,

such that said grid is negative when the anode of said device is positive upon said rectifying.

means being rendered inoperative.

3. Apparatus according to claim 1 in which said temperature responsive resistor has ,a negative temperature coecient of resistance.

4. Apparatus according to claim 1 in which said current switching means is a saturab-le reactor.

5. Apparatus according to claim 1, including said medium adapted to be heated, and in which said medium is a blanket having a heating means associated therewith, said heating means being connected to a source of current through said current switching means.

RAYMOND A. YORK. HARRY J. WOLL.

10 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,694,264 Hull Dec. 4, 1928 2,086,966 Shrader July 13, 1937 2,189,462 Donle et al Feb. 6, 1940 2,193,578 Bruce Mar. 12, 1940 2,266,569 Schneider et al Dec. 16, 1941 2,278,633 Bagnall Apr. 7, 1942 2,349,849 Deal May 30, 1944 f 2,429,453 Crowley Oct. 21, 1947 

